The present invention relates to sports rackets in general, but more specifically to a tennis racket having a hitting area strung with longitudinal and transverse (cross) strings and having a frame surrounding and supporting the string network.
It is well known that when a ball hits the network, shock is introduced through the string network to the frame following the impact. The excitation will terminate after the damping of the material consumes the residual dynamic energy in the string network and the frame.
However, people reasonably familiar with the art often confuse vibration with shock even though these are two opposing dynamic phenomena. Vibration is generally characterized by continuous harmonic or random motion of relatively small acceleration amplitude. On the other hand, shock commonly features a sharp sudden change in velocity or from a static condition due to a sudden acceleration from an abruptly applied force on the structure. When a tennis racket is swung against an incoming ball and returned, the contact lasts a few thousands of a second. The predominating phenomenon is not the velocity of the racket at any point in time, but rather, it is the extremely large velocity change per unit time-acceleration which the player feels. It is the shock passing through the string network.
To reduce shock, the prior art devised means to increase damping and to accelerate the energy consumption. One category of damping devices is the use of viscoelastic material in the form of small ball-like or strip objects attached to the string network which vibrate together with the string and achieve the purpose of hastening the energy consumption. In this category, U.S. Pat. No. 4,180,265 discloses a device inserted between the space provided by two parallel strings and the frame which supports them, wherein the device mechanically interlocks the parallel strings. Another example is disclosed in U.S. Pat. No. 4,776,590, which provides a block comprising two different foam materials inserted between parallel neighboring strings. U.S. Pat. No. 4,609,194 discloses a block of three portions inserted between parallel strings; and U.S. Pat. No. 4,575,083 discloses damping strips which border the edge regions of the hitting area and are sandwiched between perpendicular strings to assure that the strips vibrate together with the vibrating string. There are prior art disclosures of damping devices attached to the frame such as in U.S. Pat. Nos. 4,765,620; 4,600,194 and 4,634,124 which are not directly fastened to the strings, but rather, are attached to frames.
In still other patents in the prior art, use is made of a block of mass, such as rubber, fastened on the network near the frame as a means of damping to suppress the shock. As is commonly known from the theory of vibration, amplitude may be reduced to some extent by having such a device. However, whatever extent of the reduction of amplitude is possible, it is not because the dynamic energy has been consumed, but rather, that the up and down movement of the string has to carry the extra mass up and down also, and consequently, the amplitude has to be reduced. Nevertheless, in this case, the energy is not really consumed, and frequency would remain the same. In time, rubber can experience rapid increase in material stiffness and will quickly lose its ability to absorb amplitude while transmitting. On the other hand, viscoelastic and thermoplastic materials would produce better results in this regard.
Therefore, it is the principal object of the present invention to enhance the capability of a damping device for a sports racket.
Another object of the present invention is to simplify the application of a damping device to a sports racket which will permit variations of the amount of damping to be applied, and which can be readily removed and replaced when worn or broken.
A further object of the present invention is to utilize a damping device for the string network of a sports racket which is capable of assuming a variety of forms and arrangements of applications to the network strings.
Still another object of the invention is to increase the number of points of application of a damping device significantly, thereby increasing the opportunities to select those few strings most responsible for transmitting shock.
The present invention has been devised to overcome the above described deficiencies of the prior art. It comprises an elastic and flexible band made from thermoplastic material attached to one or more strings in the string network of a sports racket. Attachment to the string may be made by adhesive material and relative to a string of the network adjacent only one edge thereof, or nearer to the edge of the racket than its center. For more enhanced damping, the band may be extended to more than one string, or to a string adjacent the head portion of the racket. For maximum damping action, the band may be spirally applied in a complete circuit around the network. The band itself may take different forms and may be wound around a racket string in accordance with one or more different pitches of spiral orientation.
The present invention employs a long piece of elastic and flexible band of suitable cross-sectional shape and material with damping property such as, but not limited to, thermoplastics. The band is wrapped around and along strings characterized so that the band can move together with the string to which it is attached. The wrapping is arranged not just at junction points where longitudinal and lateral strings meet but along the entire length between two adjacent junction points. This continuity in moving together along the length between junction points is made possible by several ways which will be described below. The band is placed along a length of the string or strings adjacent to or at a slight distance from the boundary of the frame.
As distinguished from the prior art, the damping device in the form of a band is continuously attached to the string, relatively inseparable from each other due to wrapping spirally around the string or by gluing to it at certain points. The band bends and stretches together with the string when the latter bends and stretches following the impact of the ball. According to damping theory, this attachment to a string and its ability to bend and stretch are very important for a viscoelastic material to consume the dynamic energy of the vibrating string. The band is thin and is not obstructive; consequently, it does not add significantly either in weight or in air resistance even if it is used extensively lengthwise around the boundary of the frame. Damping can be achieved by placing the band on a few of the selected strings near the frame, or along the complete inner circumference of the frame to isolate the playing area of the string network from transmitting shock to the frame in any direction.